The present invention relates to semiconductor devices, and in particular the present invention relates to the formation of shallow abrupt junctions in semiconductor devices.
Since the advent of semiconductor devices, the minimum feature size has continuously decreased. In order to further decrease the minimum feature size of semiconductor devices, which are already in the submicron realm, it is important to not only reduce the horizontal dimensions of the devices, but to also reduce the vertical dimensions, i.e., junction depth.
One known method to form shallow junctions is implanting dopant ions, such as boron, phosphorous, or arsenic into an amorphous region in a silicon substrate. A subsequent high temperature rapid thermal anneal, e.g., 1100xc2x0 C. for 1 second, is then used to activate the dopant while minimizing dopant redistribution.
The high temperature anneal activates all or nearly all the dopant. Thus, the depth of the junction is a function of the depth that the dopant is implanted into the substrate, which is controlled by the energy of the implant. Consequently, it is important to use a low energy implant to maintain a shallow junction in conventionally formed devices. Unfortunately, current implanters are somewhat limited on the minimum implant energy available, and thus the depth of junctions is correspondingly limited. Because of the limits on the minimum energy available to implanters, it is particularly difficult to implant light elements, such as boron, to shallow depths. Moreover, the subsequent high temperature anneal causes further diffusion of the dopant atoms further deepening the junction.
Accordingly, there is a need for a shallow abrupt junction and a process of forming such a junction without requiring a corresponding decrease in the minimum energy produced by implanters.
The present invention provides a shallow abrupt junction formed in a single crystal substrate and a process for its preparation. A single crystal substrate is implanted with a heavy inert species, such as germanium or silicon, to form an amorphous layer at the surface of the substrate. A dopant species having a first conductivity type, such as n-type dopant, is also implanted into the substrate so that there is a high concentration of the dopant species near the surface of the substrate. A low temperature anneal, at approximately 600xc2x0 C., regrows the amorphous layer through solid phase epitaxy, which also activates the dopant species that is within the amorphous layer. The majority of any dopant species that was implanted deeper than amorphous layer is left unactivated. Thus, an abrupt junction is formed, which has a depth that is controlled by the depth of the original amorphous layer. Where the substrate has a background concentration of a dopant of a second conductivity type, such as p-type dopant, comparable to that of the implanted first conductivity type specie an abrupt pn junction is formed. Subsequent thermal processing at temperatures greater than the temperature used in the anneal will deactivate some of the dopant species and cause the dopant species to diffuse. Consequently, subsequent processing of the substrate has a thermal budget that is approximately equal to or less than the temperature used to regrow the amorphous layer.
The dopant species may be implanted to a depth that is greater than the depth of the amorphous layer. However, because only a partial activation of the dopant occurs, i.e., any dopant that is outside the amorphous layer is not activated, the depth of the junction can be shallow. The junction depth is advantageously not controlled strictly by the energy of the dopant implant, but rather is determined by the depth of the amorphous region implant. This is particularly advantageous because it is easier to control the depth of the implant of heavy ions, such as germanium, than light ions, such as boron. Moreover, because only a partial activation of the dopant occurs, by placing a high concentration of the implant within the amorphous regions, a steep dopant profile will result after the low temperature anneal.
Partial or full activation of an implanted dopant profile (depth wise rather than percentage of dopant at a given depth), without dopant diffusion, is advantageous because it provides a shallow junction that is otherwise unavailable through standard implantation and activation arrangements. In particular this is practical for boron dopants because inert species, such as silicon and germanium, that are heavier than boron can form an amorphous surface layer that is more shallow than the depth of the boron profile obtained at the minimum implantation energy available.
The formation of the shallow abrupt junction may be used advantageously in the formation of source and drain extensions, as well as any other desirably shallow junctions, such as an emitter junction in a bipolar transistor or diode.